Microbial power generator

ABSTRACT

The power generation efficiency of a microbial power generator is increased by using an easy and inexpensive unit. Two plate-like cation-exchange membranes are disposed in parallel in a tank. This arrangement allows an anode chamber to be formed between the cation-exchange membranes. Two cathode chambers are separated from the anode chamber by using the respective ion-permeable nonconductive membranes. An oxygen-containing gas is made to pass through the cathode chamber. An anode solution is supplied to the anode chamber, and, preferably, the anode solution is made to circulate. A biologically treated exhaust gas is used as the oxygen-containing gas to be supplied to the cathode chamber. Carbon dioxide in the biologically treated exhaust gas can promote transport of Na +  and K +  ions, and water vapor can increase the ion permeability, thereby increasing the power generation efficiency.

FIELD OF INVENTION

The present invention relates to a power generator utilizing a microbialmetabolic reaction. In particular, the present invention relates to apower generator which utilizes, as electrical energy, reducing power asobtained during microbial oxidative decomposition of organic matter.

BACKGROUND OF INVENTION

Recently, there has been an increased need for a method for generatingelectricity, the method taking the global environment intoconsideration. Technological development in microbial power generationhas also been progressing. The term “microbial power generation” refersto a power generation method in which electrical energy is extracted,the electrical energy being obtained when microorganisms utilize organicmatter.

Generally, microorganisms, organic matter utilized by themicroorganisms, and electron carriers (electron mediator) are made tocoexist in an anode chamber where an anode is disposed for microbialpower generation. The electron mediator enters the microorganism,receives electrons generated by microbial oxidation of organic matter,and transfers the electrons to the anode. The anode is electricallyconnected to a cathode via an external resistor (load). The electronswhich have been transferred to the anode move to the cathode via theexternal resistor (load), and then are transferred to an electronacceptor that contacts the cathode. Such electron transfer allowscurrent to flow between the cathode and the anode.

For microbial power generation, since the electron mediator directlyextracts electrons from a microorganism, theoretical energy conversionefficiency is high. However, actual energy conversion efficiency is low,and an improvement in power generation efficiency is sought. Therefore,in order to increase the power generation efficiency, variousconsiderations and developments have been made with respect to electrodematerials and structures, types of electron mediator, selection ofmicrobial species, and the like (see, for example, Patent Documents 1and 2).

Patent Document 1 describes that: a cathode chamber and an anode chamberare divided by an alkali-ionic conductor made of a solid electrolyte;the spaces inside of the cathode chamber and the anode chamber arefilled with a phosphate buffer (a buffer) having pH 7; and air is blowninto the phosphate buffer (a cathode solution) in the cathode chamber togenerate power.

Patent Document 2 describes that: a porous body is installed as acathode plate in such a manner as to cause the porous body to contact anelectrolyte membrane which divides a cathode chamber from an anodechamber; air is distributed in the cathode chamber; and the air is madeto contact a solution in a space of the porous body. Hereinafter, thecathode which utilizes oxygen in air as an electron acceptor while airis distributed in a cathode chamber in such a manner may be referred toas an “air cathode”.

The generator has advantages as follows: a microbial power generatorutilizing the air cathode does not require a cathode solution; thegenerator also only requires to simply distribute air in the cathodechamber; and aeration of the cathode solution is not necessary.

Previously, in order to increase the power generation efficiency of amicrobial power generator using the air cathode, the following wereconsidered:

1) mediators for the anode (see, for example, Patent Document 3);2) pH adjustment of the anode chamber;3) types of cathode catalyst and methods for supporting the activeingredient of the catalyst; and4) cathode structures.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Publication No. 2000-133326A

Patent Document 2: Japanese Patent Publication No. 2004-342412A

Patent Document 3: Japanese Patent Publication No. 2006-331706A

SUMMARY OF INVENTION Object of the Invention

Conventional microbial power generators have a low power generationefficiency from 50 to 150 W/m³ per 1 m³ of the anode, therefore afurther increase in the power generation efficiency has been desired.

It is an object of the present invention to provide a method formicrobially generating electricity and a microbial power generator,which can increase the power generation efficiency of the microbialpower generator by simple and inexpensive means.

Means for Attaining the Object

A method of a first embodiment for microbially generating electricity byusing a microbial power generator which includes: an anode chamberhaving an anode and retaining a solution containing a microorganism andan electron donor; and a cathode chamber having a cathode which contactsan ion-permeable nonconductive membrane, the ion-permeable nonconductivemembrane separating the cathode chamber from the anode chamber,comprises a step of generating power by feeding an oxygen-containing gasto the cathode chamber, wherein the oxygen-containing gas includes abiologically treated exhaust gas.

A method of a second embodiment for microbially generating electricityis characterized in that, in the first embodiment, an aerobicbiologically treated exhaust gas is fed to the cathode chamber as theoxygen-containing gas.

A method of a third embodiment for microbially generating electricity ischaracterized in that, in the first embodiment, air and an anaerobicbiologically treated exhaust gas are fed to the cathode chamber as theoxygen-containing gas.

A method of a fourth embodiment for microbially generating electricityby using a microbial power generator which includes: an anode chamberhaving an anode and retaining a solution containing a microorganism andan electron donor; and a cathode chamber having a cathode which contactsan ion-permeable nonconductive membrane, the ion-permeable nonconductivemembrane separating the cathode chamber from the anode chamber,comprises a step of generating power by feeding an oxygen-containing gasto the cathode chamber, wherein carbon dioxide and water vapor arecontained in the oxygen-containing gas to be fed to the cathode chamber.

A method of a fifth embodiment for microbially generating electricity ischaracterized in that, in the fourth embodiment, air is made to flowthrough a water tank for aeration so as to contain water vapor for theair, and then the air is fed to the cathode chamber with carbon dioxide.

A microbial power generator of a sixth embodiment comprises: an anodechamber having an anode and retaining a solution containing amicroorganism and an electron donor; a cathode chamber having a cathodewhich contacts an ion-permeable nonconductive membrane, theion-permeable nonconductive membrane separating the cathode chamber fromthe anode chamber; and means for feeding an oxygen-containing gas to thecathode chamber, wherein the generator is provided with means forfeeding a biologically treated exhaust gas into the cathode chamber.

A microbial power generator of a seventh embodiment is characterized inthat, in the sixth embodiment, the generator has means for feeding anaerobic biologically treated exhaust gas into the cathode chamber.

A microbial power generator of a eighth embodiment is characterized inthat, in the sixth embodiment, the generator has means for feeding airand an anaerobic biologically treated exhaust gas into the cathodechamber.

A microbial power generator of a ninth embodiment comprises: an anodechamber having an anode and retaining a solution containing amicroorganism and an electron donor; a cathode chamber having a cathodewhich contacts an ion-permeable nonconductive membrane, theion-permeable nonconductive membrane separating the cathode chamber fromthe anode chamber; and means for feeding an oxygen-containing gas to thecathode chamber, wherein the generator is provided with means for makingcarbon dioxide and water vapor be contained in the oxygen-containing gasto be fed to the cathode chamber.

A microbial power generator of a tenth embodiment is characterized inthat, in the ninth embodiment, the means for making carbon dioxide andwater vapor be contained in the oxygen-containing gas is means formaking air flow through a water tank for aeration so as to contain watervapor for the air and then feeding the air with carbon dioxide to thecathode chamber.

Effects of Invention

In the present invention, by using simple and inexpensive means by whicha biologically treated exhaust gas is fed into the cathode chamber, thepower generation efficiency of the microbial power generator isenhanced.

The inventors have conducted intensive research for increasing the powergeneration efficiency of the microbial power generator. As a result, theinventors have found that adding an acidic gas into an oxygen-containinggas to be fed to the cathode chamber promotes transport of Na⁺ and K⁺ions through the ion-permeable nonconductive membrane by a pHneutralizing effect due to the acidic gas, so that the power generationefficiency can be increased. The inventors filed a patent application(Japanese Patent Application No. 2008-280104 (hereinafter, referred toas “the preceding application”)) regarding this. Use of carbon dioxideas the acidic gas is preferable because carbon dioxide is inexpensive,very safe, and does not have a problem of corroding equipment.

Further considerations have been made on the basis of the precedingapplication. As a result, the inventors have found that adding watervapor into the oxygen-containing gas to be fed to the cathode chamberenables the power generation efficiency to be further markedly enhanced.

Details of the action mechanism responsible for the effect of increasingpower generation efficiency by this water vapor remain unclear. However,the water vapor is presumed to promote ion permeation of anion-permeable nonconductive membrane. That is, it is known that anion-permeable nonconductive membrane such as an ion-exchange membranevaries in its ion permeability according to water content thereof, andthat a low water content decreases the ion permeability. When ananion-exchange membrane is used for the ion-permeable nonconductivemembrane in particular, dissociation of water at a cathode is requiredfor the ion permeation during biological power generation. A certainamount of water supply to a cathode chamber seems to effectively act notonly on the ion permeation but also on production of a hydroxy-ion.Consequently, introducing water vapor into the cathode chamber iseffective in dissociation of water on the cathode.

The inventions according to the fourth and fifth embodiments and theninth and tenth embodiments have been achieved on the basis of suchfindings.

In such a manner, adding carbon dioxide and water vapor into theoxygen-containing gas to be fed to the cathode chamber can markedlyenhance the power generation efficiency. However, sites (e.g., awastewater treatment plant and a raw garbage treatment plant) whichcarry out biological power generation usually fail to have sources thatsupply carbon dioxide. In addition, liquid carbon dioxide, etc., isexpensive, and therefore economically disadvantageous. Moreover,humidification for addition of water vapor is troublesome.

In contrast, a biologically treated exhaust gas such as a biologicallytreated exhaust gas generated by an activated sludge method containssufficient oxygen, and further contains carbon dioxide generated duringwastewater treatment. So, the concentration of carbon dioxide is high.Moreover, the exhaust gas has high humidity and includes a sufficientamount of water vapor.

In particular, when an aeration tank having activated sludge is operatedat neutral to weakly acidic pH, the exhaust gas has a higherconcentration of carbon dioxide, and is suitable as a gas to be fed tothe cathode chamber.

In addition, use of an air diffuser having high oxygen-dissolutionefficiency such as a microscopic-bubble-diffuser tube which has beenrecently widely used for the purpose of energy saving enables theconcentration of carbon dioxide in the exhaust gas to increase, and ismore preferable.

In contrast, an anaerobic biologically treated exhaust gas does notcontain oxygen, but has a high concentration of carbon dioxide, highhumidity, and a high water vapor content. Accordingly, even for theanaerobic biologically treated exhaust gas, blending with anoxygen-containing gas such as air allows the exhaust gas to beeffectively used as a gas to be fed to the cathode chamber.

In view of the above, on sites which carry out biological powergeneration by using wastewater or organic waste as an energy source, abiological treatment facility which emits a biologically treated exhaustgas is preferably built close to the biological-power-generationfacility, which is advantageous from an aspect of gas transportation.

The inventions according to the first to third embodiments and the sixthto eighth embodiments have been achieved on the basis of such findings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional schematic view of a microbial powergenerator according to an embodiment of the present invention.

FIG. 2 shows a cross-sectional schematic view of a microbial powergenerator according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, by referring to the drawings, embodiments of the method formicrobially generating electricity and the microbial power generator ofthe present invention are described in detail.

FIG. 2 shows a schematic cross-sectional view indicating a configurationoutline of the method for microbially generating electricity and thepower generator according to the present invention.

In a microbial power generator as shown in FIG. 2, the inside space of atank 1 is compartmentalized into a cathode chamber 3 and an anodechamber 4 by an ion-permeable nonconductive membrane 2. In the cathodechamber 3, a cathode 5 is disposed so as to contact the ion-permeablenonconductive membrane 2.

In the anode chamber 4, an anode 6 made of a conductive porous materialis disposed. The anode 6 directly contacts the ion-permeablenonconductive membrane 2 or contacts the membrane 2 via about one to twolayers of microbial film. When the ion-permeable nonconductive membrane2 is a cation-permeable membrane, protons (H⁺) can be transferred to theion-permeable nonconductive membrane 2 from the anode 6.

The inside space of the cathode chamber 3 is vacant. Anoxygen-containing gas (i.e., an aerobic biologically treated exhaust gasin the present embodiment) is introduced from a gas inlet 7 into thecathode chamber 3, and an exhaust gas flows out therefrom via a gasoutlet 8 to pass through an ejection pipe 25.

As described below, a cation-permeable membrane is preferable for theion-permeable nonconductive membrane 2 which separates the cathodechamber 3 from the anode chamber 4. However, other materials can beused.

Microorganisms are supported on the anode 6 made of a porous material.An anode solution L is fed into the anode chamber 4 from an inlet 4 a.Waste fluid flows out from an outlet 4 b. The inside of the anodechamber 4 is kept anaerobic.

The anode solution L in the anode chamber 4 is made to circulate througha circulation efflux port 9, a circulation pipe 10, a circulation pump11, and a circulation return port 12. Partway along the circulation pipe10, a pH meter 14 that determines the pH of a solution efflux from theanode chamber 4 is installed, and a pipe 13 used for adding an alkalisuch as aqueous sodium hydroxide is connected. Then, the alkali is addedas needed so as to set the pH of the anode solution L to 7 to 9.

Condensed water generated in the cathode chamber 3 is drained from acondensed water outlet which is not shown in the Figure.

Electromotive force occurring between the cathode 5 and the anode 6allows current to flow through an external resistor 21 via terminals 20and 22.

An aerobic biologically treated exhaust gas containing oxygen, carbondioxide, and water vapor is made to flow through the cathode chamber 3,and, by running a pump 11 to circulate the anode solution L as needed, areaction:

(Organic matter)+H₂O→CO₂+H⁺ +e ⁻

proceeds in the anode chamber 4. This electron e⁻ flows through theanode 6, the terminal 22, the external resistor 21, and the terminal 20to the cathode 5.

The proton H⁺ which has been generated in the above reaction moves tothe cathode 5 by passing through a cation-permeable membrane of theion-permeable nonconductive membrane 2. At the cathode 5, a reaction:

O₂+4H⁺+4e ⁻→2H₂O

proceeds. H₂O produced by this reaction at the cathode becomes condensedto yield condensed water. K⁺ and Na⁺, etc., which permeate through acation-permeable membrane of the ion-permeable nonconductive membrane 2are dissolved in this condensed water. For a conventional microbialpower generator through which air passes as an oxygen-containing gas,the condensed water has a high alkaline pH from about 9.5 to 12.5 due tothe above. However, in the present invention, since a biologicallytreated exhaust gas containing carbon dioxide passes through agenerator, the neutralizing effect due to carbon dioxide allows the pHof this condensed water to be kept between about 7.5 and 9.

For example, when a cation-permeable membrane is used as theion-permeable nonconductive membrane 2, electrons generated at the anode6 flow from the terminal 22, the external resistor 21, and the terminal20 to the cathode 5. In the meantime, protons as well as Na⁺ and K⁺ inthe anode solution L which are introduced into the anode 6 permeatethrough the cation-permeable membrane of the ion-permeable nonconductivemembrane 2 to be transported to the cathode chamber 3. In this case, apH-neutralizing effect caused by inclusion of carbon dioxide in the gaswhich passes through the cathode chamber 3 is presumed to promote thetransport of Na⁺ and K⁺. Because of this, an increase in the powergeneration efficiency can be achieved.

For the permeation of the proton H⁺, K⁺, and Na⁺ through theion-permeable nonconductive membrane 2, the transport of these ions canbe promoted by increasing the ion permeability of the ion-permeablenonconductive membrane 5A because a sufficient amount of water vapor isfed into the cathode chamber 3. This leads to much further improvementin the power generation efficiency.

In the anode chamber 4, generation of CO₂ by a water-decomposingreaction by microorganisms tends to lower the pH. Then, an alkali isadded to the anode solution L so as to keep the pH preferably between 7and 9, the pH being detected by the pH meter 14. This alkali may bedirectly added to the anode chamber 6, but by adding the alkali to thecirculating water, the whole region inside the anode chamber 6 can bemaintained to have a pH between 7 and 9 without uneven distribution.

FIG. 1 shows a cross-sectional overview of a microbial power generatoraccording to a particularly preferred embodiment of the presentinvention.

With respect to a microbial power generator as shown in FIG. 1, twoplate-like ion-permeable nonconductive membranes 31 and 31 are disposedin parallel in a substantially rectangular tank 30. This arrangementallows an anode chamber 32 to be formed between the ion-permeablenonconductive membranes 31 and 31. Two cathode chambers 33 and 33 areseparated from the anode chamber 32 by using the respectiveion-permeable nonconductive membranes 31.

Inside the anode chamber 32, an anode 34 made of a porous material isdisposed so as to directly contact the respective ion-permeablenonconductive membranes 31 or to contact them via about one to twolayers formed of a biological membrane. It is preferable that the anode34 is lightly pressed onto the ion-permeable nonconductive membranes 31and 31 by applying a pressure of, for example, 0.1 kg/cm² or less.

In a cathode chamber 33, a cathode 35 is disposed so as to contact theion-permeable nonconductive membrane 31. This cathode 35 is pressed ontothe ion-permeable nonconductive membrane 31 by using packing 36. Inorder to enhance adhesion between the cathode 35 and the ion-permeablenonconductive membrane 31, both may be welded or bonded using adhesive.

A biologically treated exhaust gas as an oxygen-containing gas isintroduced into the space between the cathode 35 and the tank 30, andflows therethrough.

The cathode 35 and the anode 34 are connected to an external resistor 38via terminals 37 and 39, respectively.

An anode solution L is introduced into the anode chamber 32 from aninlet 32 a, and waste fluid flows out from an outlet 32 b. The inside ofthe anode chamber 32 is kept anaerobic.

The anode solution in the anode chamber 32 is made to circulate througha circulation efflux port 41, a circulation pipe 42, a circulation pump43, and a circulation return port 44. An oxygen-containing gas from apipe 61 flows in the respective cathode chambers 33 through a gas inlet51, and an exhaust gas flows out from a gas outlet 52 to pass through apipe 63. An aerobic biologically treated exhaust gas is used as anoxygen-containing gas in the present embodiment.

Partway along a circulation pipe 42 for the anode solution, a pH meter47 is installed, and a pipe 45 used for adding an alkali is connected.The pH of the anode solution that flows out from the anode chamber 32 isdetected by the pH meter 47. An alkali such as aqueous sodium hydroxideis added so as to produce a pH preferably between 7 and 9.

For a microbial power generator as shown in FIG. 1, an aerobicbiologically treated exhaust gas containing oxygen, carbon dioxide, andwater vapor is fed to the cathode chamber 33. The anode solution is madeto pass through the anode chamber 32, or the anode solution ispreferably made to circulate through the anode chamber 32. As a result,a potential difference occurs between the cathode 35 and the anode 34,and current flows through the external resistor 38.

The following is a detailed description of a microorganism and an anodesolution for this microbial power generator, a biologically treatedexhaust gas, an ion-permeable nonconductive membrane, preferablematerials used for the anode and cathode, and the like.

A microorganism that produces electrical energy by being included in theanode solution L is not particularly limited as long as it functions asan electron donor. Examples can include bacteria, filamentous fungi,yeast, and the like which belong to each genus of Saccharomyces,Hansenula, Candida, Micrococcus, Staphylococcus, Streptococcus,Leuconostoa, Lactobacillus, Corynebacterium, Arthrobacter, Bacillus,Clostridium, Neisseria, Escherichia, Enterobacter, Serratia,Achromobacter, Alcaligenes, Flavobacterium, Acetobacter, Moraxella,Nitrosomonas, Nitorobacter, Thiobacillus, Gluconobacter, Pseudomonas,Xanthomonas, Vibrio, Comamonas, and Proteus (Proteus vulgaris).Activated sludge, as sludge containing the above microorganisms, asobtained from a biological treatment tank processing organicmatter-containing water such as sewage, microorganisms which areincluded in water efflux from a primary sedimentation tank for sewage,and anaerobic digestive sludge, etc., are fed to the anode chamber as aninoculum. This allows the microorganisms to be retained at the anode. Inorder to enhance power generation efficiency, it is preferable to retaina high concentration of microorganisms in the anode chamber, and themicrobial concentration is preferably, for example, between 1 and 50g/L.

As the anode solution L, a solution which maintains microorganisms orcells and has compositions necessary for power generation is used. Forexample, when power generation utilizing a respiratory system is carriedout, a medium having compositions such as nutrients and energy sourcesrequired for carrying out respiratory metabolism can be used, includinga bouillon medium, M9 medium, L medium, a malt extract, MY medium, anitrifying-bacteria-selection medium, and the like as a solution for theanode side. In addition, organic waste such as sewage, organicindustrial drainage, and raw garbage can be used.

In the anode solution L, an electron mediator may be included so as toreadily withdraw electrons from microorganisms or cells. Examples of theelectron mediator can include, for example, compounds having a thionineskeleton such as thionine, dimethyl sulfonated thionine, new methyleneblue, and toluidine blue-O; compounds having a2-hydroxy-1,4-naphthoquinone skeleton such as2-hydroxy1,4-naphthoquinone; Brilliant Cresyl Blue, Gallocyanine,Resorufin, Alizarin Brilliant Blue, phenothiazinone, phenazineethosulfate, safranin-O, dichlorophenolindophenol, ferrocene,benzoquinone, phthalocyanine, or benzyl viologen and derivativesthereof; and the like.

In order to further efficiently yield electric power, dissolution ofmaterials which increase the power generation function of amicroorganism such as an antioxidant like vitamin C, and afunction-increasing material which only activates a specific electrontransport system and substance transport system in a microorganism arepreferable.

The anode solution L may include a phosphate buffer depending on need.

The anode solution L contains organic matter. This organic matter is notparticularly limited if degraded by microorganisms. Examples of theorganic matter that can be used include, for example, water-solubleorganic matter, microparticles made of organic matter that are dispersedin water, and the like. The anode solution may be organic wastewatersuch as sewage and drain water from a food factory. The concentration oforganic matter in the anode solution L is preferably a highconcentration of between about 100 and 10,000 mg/L so as to enhancepower generation efficiency.

A biologically treated exhaust gas which is made to pass through acathode chamber may be, as described above, an aerobic biologicallytreated exhaust gas containing oxygen, carbon dioxide, and water vapor.Alternatively, it may be an anaerobic biologically treated exhaust gascontaining carbon dioxide and water vapor. When the anaerobicbiologically treated exhaust gas is used as a biologically treatedexhaust gas, the biologically treated exhaust gas can be mixed with anoxygen-containing gas such as air in an appropriate ratio of, forexample, the anaerobic biologically treated exhaust gas:air=1:0.5 to 500(ratio by volume). Alternatively, instead of air, an aerobicbiologically treated exhaust gas may be mixed.

The source of the biologically treated exhaust gas is not particularlylimited. As well as an exhaust gas from an activated sludge method, anygas can be used as long as it has been exhausted during variousbiological treatments such as fixed bed, fluidized-bed, nitrification,denitrification, and compost, and it has a higher carbon dioxideconcentration than air. In addition, two or more kinds of thesebiologically treated exhaust gases can be blended to be used.

As described above, an exhaust gas from an aeration tank which holdsactivated sludge and is being operated at neutral pH to weakly acidic pHis preferable, since the exhaust gas contains carbon dioxide at a highconcentration. In addition, an exhaust gas from an aeration tank usingan air diffuser having high oxygen-dissolution efficiency such as amicroscopic-bubble-diffuser tube which has been recently widely used forthe purpose of energy saving is also preferable because of a highconcentration of carbon dioxide.

A composition of a biologically treated exhaust gas varies widelydepending on where it is produced. However, an aerobic biologicallytreated exhaust gas usually has a composition where an O₂ concentrationis between 15 and 19% by volume, a CO₂ concentration is between 1 and 5%by volume, and humidity is between about 95 and 100%. An anaerobicbiologically treated exhaust gas usually has a composition where a CO₂concentration is between 20 and 40% by weight, and humidity is betweenabout 95 and 100%.

As described above, in one embodiment of the generating method by usingthe microbial power generator in which an oxygen-containing gas such asair is fed to the cathode chamber according to the present invention,carbon dioxide and water vapor can be added to the oxygen-containinggas. Such a method can be carried out by adding carbon dioxide into airas the oxygen-containing gas to have a flow ratio of air:carbondioxide=100:about 0.1 to 20 and by injecting water vapor thereto. Watervapor can be added to the gas by, instead of injecting thereto, makingthe gas flow through a water tank to perform aeration. In this case, thegas becomes to contain water vapor under saturated vapor pressure at thetemperature.

The exhaust gas from the cathode chamber may be subjected todeoxygenation treatment depending on need. Then, the exhaust gas is madeto pass through the anode chamber, and can be used for purging dissolvedoxygen from the anode solution L.

The ion-permeable nonconductive membrane may be an ion-permeablemembrane such as a nonconductive ion-permeable cation-permeable membraneor anion-permeable membrane. Various ion-exchange membranes or reverseosmosis membranes, etc. can be employed. A cation-exchange membrane oranion-exchange membrane which has high proton selectivity is preferablyused as an ion-exchange membrane. Examples of the cation-exchangemembrane that can be used include, for example, Nafion® manufactured byDu Pont, Inc., CMB membrane which is a cation-exchange membranemanufactured by ASTOM Corporation, and the like. In addition, examplesof the preferable anion-exchange membrane include an anion-exchangemembrane manufactured by ASTOM Corporation, an anionic electrolytemembrane manufactured by TOKUYAMA Corporation, and the like. Theion-permeable nonconductive membrane is preferably thin and tough.Usually, the membrane thickness is preferably between 30 and 300 μm, andparticularly preferably between 30 and 200 μm.

As the ion-permeable nonconductive membrane, a cation-exchange membraneis preferably used because the present invention effectively exerts aneffect due to injection of carbon dioxide. In addition, with respect tothe effect of increasing ion permeability by water vapor, ananion-exchange membrane is preferably used.

In order to retain a large number of microorganisms, the anode ispreferably a porous body which has a large surface area, forms a largenumber of spaces, and possesses water permeability. Specifically,examples can include a sheet made of a conducting substance, the sheetbeing made rough at least on its surface, and a porous conductor inwhich a conducting substance is formed into a felt-like or other poroussheet (e.g., graphite felt, foamed titanium, foamed stainless).

Abutting such a porous anode to the ion-permeable nonconductive membranedirectly or via microbial layers enables electrons generated during amicrobial reaction to pass to the anode without using an electronmediator, and renders the electron mediator unnecessary.

A plurality of sheet-like conductors may be stacked to form an anode. Inthis case, the same type of conductor sheet can be stacked.Alternatively, different types of conductor sheets (e.g., graphite feltand a graphite sheet having a rough surface) can be stacked on oneanother.

The anode preferably has a total thickness from 3 mm or more to 40 mm orless, and particularly between about 5 and 20 mm. When stacked sheetsconstitute an anode, in order to pass liquid along with a paired surface(a lamination plane) between the sheets, the lamination plane ispreferably oriented in a direction from an inlet to an outlet.

In the present invention, an anode chamber may be divided into aplurality of compartments. Serial connection of each compartment allowsa decrease in the pH of each compartment to be reduced, and the pH ofthe liquid in the anode chamber may then be adjusted. Division of theanode chamber can decrease the amount of decomposition of organic matterin each compartment. As a result, the amount of production of carbondioxide decreases, so that a decrease in the pH of each compartment canbe lowered.

A cathode preferably has a conductive base material and anoxygen-reducing catalyst which is supported on the conductive basematerial.

For a conductive base material, any material can be used as long as ithas a high conductivity, high corrosion resistance, sufficientconductivity and corrosion resistance in the case of a thin thickness,and mechanical strength as a conductive base material. Examples of thematerial that can be used include, but are not limited to, graphitepaper, graphite felt, graphite cloth, stainless mesh, titanium mesh, andthe like. Among them, from a viewpoint of durability and easy processingin particular, a graphite-based base material such as graphite paper,graphite felt, or graphite cloth is preferable, and graphite paper isparticularly preferable. In addition, these graphite-based basematerials may be those hydrophobized by a fluorine resin such aspolytetrafluoroethylene (PTFE).

Regarding the thickness of the conductive base material of the cathode,a large thickness impairs oxygen permeability, and a small thicknessfails to satisfy the required properties such as strength necessary forthe base material. Thus, the thickness is preferably between about 20and 3000 μm.

As an oxygen-reducing catalyst, as well as a noble metal such asplatinum, a metal oxide such as manganese dioxide is suitable becausethey are inexpensive and have a better catalytic activity. The supportedamount of catalyst is preferably set to about 0.01 to 2.0 mg/cm².

EXAMPLES

Hereinafter, the present invention is specifically illustrated byreferring to Examples and Comparative Examples.

Comparative Example 1

In an anode chamber having a size of 7 cm (width)×25 cm (length)×2 cm(thickness), two sheets formed of a graphite felt having a thickness of1 cm were stacked and installed to form an anode. A cathode chamber wasformed beside this anode by interposing a cation-exchange membrane(manufactured by Du Pont, Inc.; product name is “Nafion 115”®) as anion-permeable nonconductive membrane. The cathode chamber had a size of7 cm×25 cm×0.5 cm (thickness). Next, a Pt catalyst manufactured byTanaka Kikinzoku Kogyo K. K. (Pt-supporting carbon black; Pt content is50% by weight) was dispersed in 5% by weight of a Nafion® solution(manufactured by Du Pont, Inc.). This solution was applied to carbonpaper (manufactured by Toyo Carbon Co., Ltd.) having a thickness of 160μm and being subjected to water-repellent treatment using PTFE to have aPt-supporting amount of 0.4 mg/cm². Then, the carbon paper was dried at50° C., and the resulting paper was tightly attached to the abovecation-exchange membrane as a cathode.

A stainless wire was bonded using a conductive paste to the graphitefelt of the anode and the carbon paper of the cathode, which formed anelectric leading wire. Then, a 2-Ω resistor was connected partway alongthe wire.

An anode solution containing 1000 mg/L of acetic acid, phosphoric acid,and ammonia while keeping a pH of 7.5 was made to pass through the anodechamber. This anode solution had been heated beforehand to 35° C. inanother water tank. The solution which had been heated at this watertank was made to pass through the anode chamber to have a flow rate of10 mL/min. By this procedure, the anode chamber was heated to 35° C. Itis notable that before the anode solution passed through the chamber,liquid efflux from another microbial power generator had been made topass through the chamber as an inoculum.

A cathode chamber was aerated with dry air having ordinary temperaturesto have a flow rate of 0.5 L/min.

As a result, in three days after the anode solution started passingthrough the chamber, the generation of electricity became almostconstant, and the generation of electricity per 1 m³ of the anode was140 W (i.e., power generation efficiency was 140 W/m³).

Comparative Example 2

Power generation was conducted in the same manner as Comparative Example1 except that carbon dioxide was injected into the air to be fed to thecathode chamber from a carbon dioxide cylinder at a rate of 1 mL/min(0.2% to the air). Immediately after the injection of carbon dioxide,power generation efficiency began to increase. After 5 minutes, thepower generation efficiency was 180 W/m³.

Example 1

Air to be fed to the cathode chamber was introduced into a closed 2-Lwater tank which had been filled with 1.5 L of pure water, and aerationwas performed for 4 minutes. Then, the air was fed into the cathodechamber together with carbon dioxide. Except for the above, powergeneration was carried out in the same manner as Comparative Example 2.As a result, the power generation efficiency increased to 210 W/m³. Itis notable that in this Example 1, the humidity of the air became 97% byperforming aeration of a water tank with the air.

Example 2

Instead of air, an aerobic biologically treated exhaust gas (O₂concentration: 19.8% by volume; CO₂ concentration: 1.3% by volume;humidity: 99%) was fed to the cathode chamber. The exhaust gas wasproduced by a fluidized-bed-type biological treatment tank having avolume of 40 m³ and BOD load of 0.5 kg/m³·day) in a wastewater treatmentplant of a research institute. Except for the above, power generationwas carried out in the same manner as Comparative Example 1. As aresult, a power generation efficiency of 255 W/m³ was achieved.

Example 3

Instead of air, a mixed gas containing 200 mL/min of a biogas (CO₂concentration: 32% by volume; humidity: 99%) and 400 mL/min of air wasfed to the cathode chamber. The biogas was produced by a UASB apparatus(diameter: 10 cm; height: 60 cm; methanol synthetic substrate; load: 30kg-COD_(cr)/m³/day). Except for the above, power generation was carriedout in the same manner as Comparative Example 1. As a result, a powergeneration efficiency of 248 W/m³ was achieved.

The above results demonstrate that adding of carbon dioxide and watervapor to the oxygen-containing gas to be fed to the cathode chamber oruse of a biologically treated exhaust gas as the oxygen-containing gasincreases power generation efficiency.

The present invention has been described in detail by using specificembodiments. However, it is apparent to those skilled in the art thatvarious modifications are allowed without departing from the spirit andscope of the present invention.

In addition, the present application claims benefit of Japanese PatentApplication No. 2008-327988 filed on Dec. 24, 2008, which is hereinincorporated by reference in its entirety.

REFERENCE NUMERALS

-   -   1, 30: Tank    -   2, 31: Ion-permeable nonconductive membrane    -   3, 33: Cathode chamber    -   4, 32: Anode chamber    -   5, 35: Cathode    -   6, 34: Anode

1. A microbial power generator, comprising: an anode chamber having ananode and retaining a solution containing a microorganism and anelectron donor; a cathode chamber having a cathode which contacts anion-permeable nonconductive membrane, the ion-permeable nonconductivemembrane separating the cathode chamber from the anode chamber; andmeans for feeding an oxygen-containing gas to the cathode chamber,wherein the generator is provided with means for feeding a biologicallytreated exhaust gas into the cathode chamber.
 2. The microbial powergenerator according to claim 1, wherein the generator has means forfeeding an aerobic biologically treated exhaust gas into the cathodechamber.
 3. The microbial power generator according to claim 1, whereinthe generator has means for feeding air and an anaerobic biologicallytreated exhaust gas into the cathode chamber.
 4. A microbial powergenerator, comprising: an anode chamber having an anode and retaining asolution containing a microorganism and an electron donor; a cathodechamber formed of an air cathode, and having a cathode, a first gasinlet introducing a biologically treated exhaust gas generated by anactivated sludge method containing oxygen, carbon dioxide, and watervapor, or introducing a mixed gas of an anaerobic biologically treatedexhaust gas containing carbon dioxide and water vapor, and air, to thecathode chamber, and a second gas inlet feeding an oxygen-containing gasto the cathode chamber; and an ion-permeable nonconductive membranecontacting the cathode, the ion-permeable nonconductive membraneseparating the cathode chamber from the anode chamber.
 5. The microbialpower generator according to claim 4, wherein an inside space of thecathode chamber is vacant.
 6. The microbial power generator according toclaim 4, further comprising a gas inlet port introducing theoxygen-containing gas to the cathode chamber, and a gas outlet pipehaving a gas outlet port through which the exhaust gas is dischargedfrom the cathode chamber.